Manually controlled or electro-magnetic changing valves have been hitherto used in order to change directions of flow in a fluid pressure circuit, an air pressure circuit and the like. Further, it has been known that the changing valves are automatically interchanged in accordance with external conditions, such as expansion-contraction of an air cylinder, increase of oil temperature, or the like, detected by sensors or transducers.
Such changing valves can quickly change fluid passages of a relatively high pressure circuit. On the other hand, those valves are not suitable for changing passage of low pressure and large volume flow, for example, a water duct or an air duct.